This invention relates to flexible polyurethane foams used in bedding and furniture cushions, and as components of bedding mattresses and mattress pads. Produced under vacuum conditions from certain foaming mixtures, the foams of this invention provide a low density foam with high resiliency and improved flame retardancy. Such polyurethane foams are offered as replacements for latex foams in bedding and furniture applications.
Latex has been used commercially for bedding mattress applications, primarily because latex foams generally have superior resiliency. One typical measurement for resiliency is ball rebound (ASTM D 3574), where a steel ball is dropped from a fixed height onto a foam sample and then is allowed to bounce back from the sample surface. The height on the first rebound is compared to the original height, and the percent of height in rebound is reported. A higher number indicates a more resilient material. For latex, the ball rebound ranges from 65 to 75 percent, among the highest of natural materials.
Latex is available in many grades, covering a range of density, resiliency and hardness. Hardness is typically measured as IFD (xe2x80x9cindentation force deflectionxe2x80x9d). Specifically, IFD25 is the force required to compress the foam to 25% of its original thickness or height. The higher density foam grades typically have higher IFD25 and higher ball rebound. For example, a 3.2 lb/ft3 latex has an IFD25 of 13 lb and a ball rebound of 67 percent; and a 3.5 lb/ft3 latex has an IFD25 of 15 lb and a ball rebound of 71 percent.
While latex has been popular in cushioning applications, latex also presents several major problems. First, it contains proteins that can induce allergic reactions upon contact for some people. Also, as a natural material, latex has poor flame retardancy, and thus presents a serious fire hazard. In addition, raw latex is farmed and harvested in tropical climates overseas, which requires it to be shipped over a long distance, adding substantially to the cost. Finally, the density for bedding-grade latex foam is relatively high at 3.2 lb/ft3 or above, thus making a mattress made of latex foam have a significant weight. Indeed, latex foam does not lend easily to density reduction. Therefore, the cushioning and bedding industry continues to seek lower density latex replacements.
Polyurethane foams with varying density and hardness may be formed. Tensile strength, tear strength, compression set, air permeability, fatigue resistance, support factor, and cell size distribution may also be varied, as can many other properties. Specific foam characteristics depend upon the selection of the starting materials, the foaming process and conditions, and sometimes on the subsequent processing.
Cellular polyurethane structures typically are prepared by generating a gas during polymerization of a liquid reaction mixture comprised of a polyester or polyether polyol, an isocyanate, a surfactant, catalyst and one or more blowing agents. The gas causes foaming of the reaction mixture to form the cellular structure. The surfactant stabilizes the structure.
Once the foam-forming ingredients are mixed together, it is known that the foam may be formed under either elevated or reduced controlled pressure conditions. PCT Published Patent Application WO 93/09934 discloses methods for continuously producing slabs of urethane polymers under controlled pressure conditions. The foam-forming mixture of polyol, isocyanate, blowing agent and other additives is introduced continuously onto a moving conveyor in an enclosure with two sub-chambers. The foaming takes place at controlled pressure. Reaction gases are exhausted from the enclosure as necessary to maintain the desired operating pressure. The two sub-chambers, a saw, and air tight doors are operated in a manner that allows for continuous production of slabstock polyurethane foam.
U.S. Pat. No. 5,804,113 to Blackwell, et al., shows a method and apparatus for continuously producing slabstock polyurethane foam under controlled pressure conditions in which a layer of gas surrounds the reaction mixture during free rise expansion of the reaction mixture to prevent pressure fluctuations. Blackwell generally describes foam reaction mixtures that may include a variety of polyols and isocyanates, and does not express preference for any specific combinations.
U.S. Pat. No. 4,777,186 to Stang, et al., describes a method of foaming in a pressurized chamber held above atmospheric pressure (i.e., in the range of about 0.5 to 1000 psig). In addition to the gases emitted during foaming, additional gases may be introduced into the chamber to maintain the elevated pressure during foaming. The resulting foams have a higher IFD to density ratio than those previously known in the art.
Flexible polyurethane foams with high densities in the range of 35 to 70 kg/m3 (or 2.2 to 4.4 lb/ft3) are produced by the method disclosed in U.S. Pat. No. 5,194,453 to Jourquin, et al. Polyether polyols with molecular weights in the range of 1400 to 1800 and having primary hydroxyl group content over 50% are reacted with organic isocyanates that may be TDI, MDI or mixtures of TDI with MDI. The foams may be produced by frothing the reaction mixture, or alternatively, under vacuum conditions. Support factor was not reported, although deformation tests were conducted and the foams are indicated to have improved comfort properties.
Higher density polyurethane foams (30 kg/m3 or about 1.9 lb/ft3) are produced with the polyol combinations disclosed in U.S. Pat. No. 5,668,378 to Treboux, et al. The foam-forming mixture includes 80 to 99.8 percent by weight of a high functionality polyol or polyol blend with 8 to 25 percent EO, functionality from 3.2 to 6.0 and an equivalent weight of 1,000 to 4,000, a minor portion of a graft polyol, and an organic isocyanate that preferably is a mixture of TDI. The foams are foamed at atmospheric pressure.
U.S. Pat. No. 6,063,309 to Hager et al. discloses liquid-liquid polyol dispersions with an ethylene oxide (EO) content of 40 to 85 percent by weight. The polyols have a functionality greater than 2. The dispersion can be used to prepare hypersoft polyurethane foams.
High resiliency (HR) foams have been made commercially, but typically with ball rebound less than 60. For example, U.S. Pat. No. 6,372,812 to Niederoest et al. teaches the use of vacuum chamber pressure and MDI to obtain low density, high support foams. The Niederoest patent focuses on making foams with a density of 1.4 to 1.8 pounds per cubic foot. While the support factor was high, the ball rebound was 51, significantly below that of latex (generally above about 65).
Foams with lower density, but latex-like resiliency are continually sought for furniture, mattress components and pillows. The prior art does not disclose methods for making high ball rebound polyurethane foams at a low density. Nor does the prior art disclose such foams with improved flame retardancy.
According to the invention, flexible, high resiliency polyurethane foams are produced using a method comprising preparing a foam reaction mixture and foaming that mixture under vacuum conditions, preferably at pressures in the range of 0.6 to 0.95 bar (absolute), most preferably 0.8 to 0.95 bar (absolute). The reaction mixture contains (a) a polyol mixture of (i) about 85 to 95 percent by weight total polyols of a polyether polyol having a from 10 to 30 percent ethylene oxide groups, and having a hydroxyl number in the range of about 25 to 50 and a functionality from 2.5 to 3.5; and (ii) about 5 to 15 percent by weight total polyols of a graft polyol having a ratio of styrene to acrylonitrile of about 70 to 30, and having a hydroxyl number in the range of about 25 to 50 and a functionality from 2.5 to 3.0; (b) an isocyanate, preferably toluene diisocyanate or methylene diisocyanate, wherein the isocyanate index is in the range of 0 to 95; and (c) from about 1.5 to 2 parts per hundred parts polyol of water as a blowing agent.
Most preferably, the foam-forming composition contains up to 2 parts per hundred parts polyol of an amine catalyst, up to 2 parts per hundred parts polyol of a surfactant, up to 0.5 parts per hundred parts polyol of an organotin catalyst, and up to 2 to 6 parts per hundred parts polyol of a cross linking agent.
In addition, excellent results have been obtained using a polyol combination of (a) from 85 to 90 percent by weight total polyols of polyether polyol (functionality 3.1 to 3.3), having 15 to 20 percent EO groups and a hydroxyl number in the range of 28 to 32, and (b) from 10 to 15 percent by weight total polyols of a graft polyol having a ratio of styrene to acrylonitrile of about 70 to 30, and having a hydroxyl number in the range of about 25 to 30 and a functionality from 2.8 to 2.9 wherein the isocyanate index was in the range of 80 to 90. In this preferred embodiment, from about 2.1 to 2.8 parts per hundred parts polyol of water as a blowing agent; and up to 1.0 parts per hundred parts polyol of a surfactant are included in the reaction mixture.
The resulting polyurethane foams have densities in the range of about 2.0 to 3.0 pounds per cubic foot, with a ball rebound of above 65, and pass the flame retardancy test as outlined in the California Bureau of Home Furnishings Technical Bulletin 117 (CAL TB 117).